It is sometimes desirable to make a series of prints on a traveling web of material by the screen printing method, the web being necessarily intermittently pulled forwardly with the web stationary during each successive screen printing operation.
For example, the self-stick labels described by the Mikulas U.S. Pat. No. 4,009,065, Feb. 22, 1977, are manufactured in the form of a composite strip or web of label and protective layer materials, the web being continuously fed to an automatic punching machine which punches out the label contours while leaving the protective layer material partially punched but otherwise intact, that material being initially wider than the labels and, after punching, producing a lattice which at the punching machine, is peeled off and discarded as waste.
It is desirable to apply printing to each of the succession of labels and current demands require this printing to be done by the screen printing process which permits the application of greater printing layer thicknesses than is possible by offset or letter press printing, for example.
For screen printing a succession of closely interpositioned prints extending lengthwise on the web, the web can be intermittently pulled forwardly over a screen printing machine's printing table, the printing screen above the table being adapted to contact the web so that during each stop of the web, printing material, such as ink, can be forced through the screen by the usual knife reciprocating over the screen in the longitudinal direction of the web to spread the printing material and force it through the screen.
To obtain an acceptable production rate, the web must be pulled intermittently at an adequate rate, this requiring the knife to reciprocate rapidly. Therefore, due to the knife's stroke-end decelerating and accelerating requirements, the knife's overall reciprocating stroke length must be greater than the printing area where the printing action is desired.
The above introduced the problem that with the web intermittently pulled forwardly beneath the printing screen, a freshly made print just moved fowardly remains in contact with the screen pressed downwardly by the knife during its decelerating stroke end, thus possibly smudging or smearing the print if it remains wet at that time.
The above problem has existed for some time and various proposals have been made for solving the problem. For example, it has been proposed to lift the knife from the screen upon completion of its effective printing stroke, or to lift the screen including the knife, at that time. However, from the engineering viewpoint, such expedients are difficult to achieve and, in any event, involve high machine manufacture and maintenance costs.